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Geology of the New York Region

Read on for a journey through New York's geological time and how it has shaped the features it is most famous for today: the Adirondacks, the Catskills, Niagara Falls, the Finger Lakes, and the bedrock of New York City.

Part 1: The Geology of New York State Through Time

Basic Geologic Concepts

The crust is the uppermost and coldest layer of the Earth. Beneath it is the mantle and core. Because the crust is not particularly hot and because it experiences comparatively little pressure, the crust is relatively rigid and has broken up into a jigsaw puzzle of pieces known as tectonic plates. These tectonic plates “float” on the mantle beneath and move with the mantle’s convection currents which slowly push tectonic plates together or apart.

Two things can happen when tectonic plates are pushed together: 1) they collide, squeeze together, and push upwards to form a mountain (this is called an “orogenic event” or “orogeny” and has formed mountains like the Alps, Himalayas, and Appalachians) or 2) one slips underneath the other and melts deep underground to form magma which rises and creates a ring of volcanoes (e.g. the Cascades and the Andes). Both result in the formation of igneous and metamorphic rocks. Igneous rocks form from cooling magma which has found its way to the surface. Metamorphic rocks occur because intense heat or pressure has caused a rock’s chemical structure to change but has not caused the rock to melt. Additionally, both types of collisional events result in rock deformation such as rock folding and faulting (cracking).

New York has experienced multiple orogenic (aka. mountain-building) events throughout its geologic history. Mountains formed as tectonic plates collided and resulted in rock metamorphism, deformation, and orogeny-related volcanics (e.g. igneous intrusions). The 5 most recent orogenies are responsible for the formation of the Appalachians and the most recent orogeny occurred during the formation of Pangaea. Orogenies prior to these 5 have long since eroded away, although their metamorphosed rocks still make up the deep bedrock of New York, such as the Manhattan Prong, which lays below most of New York City’s boroughs. New York’s landscape today is, overall, the result of the formation of the Appalachians followed by millions of years of erosion and sediment deposition and, most recently, glacial activity (which led to further erosion and sediment deposition).

A time-scale of geologic history, from the inception of Earth 4.6 billion years ago to today.
Image 1: The Geologic Time Scale above is a helpful reference to use along with the information provided below. The left half shows a timeline of Earth’s geologic history which is split into Eons, Eras, Periods, and Epochs and how many millions of years ago (MYA) these occurred. The right half labels the major North American Events which includes the orogenies (mountain-building events) mentioned in this article. Events relevant to New York’s geology are outlined by a red rectangle. Geologic events are labeled with a cardinal direction (N, E, S, W) to show the region of the North American continent in which they occurred. For example, the “Grenville Orogeny (E)” occurred on the East coast.

2.5 - 1.3 billion years ago: The formation of North America

The North American continent started as a collection of microcontinents - mini volcanic islands similar to Hawaii - which formed from hotspots. These collided as a result of tectonic plate motion and accreted (joined) with one another to form the North American craton, also known as Laurentia. The “craton” makes up the interior portion of the current American continent and is the oldest and most stable part of this continent. It took hundreds of millions of years for America to accrete enough material to become the continent we know today.

As the craton was assembling from colliding microcontinents, New York’s geographic location was underwater gathering layers of sand, mud, and clay as well as calcium carbonate minerals1. These eventually compressed from the weight of water and the continued addition of sediments to form sedimentary rock such as shale (from mud and clay), limestone (from calcium carbonate minerals), and sandstone (from sand).

1.3 billion years - 650 million years ago: The Grenville Orogeny (supercontinent Rodinia)

The Adirondacks, Thousand Islands, Hudson Highlands, & Central Park

Between 1.35 and 1 billion years ago, Laurentia was involved in the formation of Rodinia: an ancient supercontinent. As part of the tectonic plate cycle, continents periodically collide and then separate. Laurentia is no exception. Laurentia collided with another continental plate in what is now called the Grenville Orogeny. The collision created a large mountain range along Laurentia’s eastern coast, similar to the Alps today. In the process, it resulted in orogeny-related volcanics and metamorphosed the pre-existing sedimentary rock into metamorphic rocks such as slate and schist (from shale), marble (from limestone), quartzite (from sandstone), and gneiss (from schist or igneous rocks; gneiss forms when a rock experiences enough heat to partially melt). Although the mountain range has long since eroded away, Grenville rocks still make up the deep bedrock of New York. These billion-year-old rocks are visible on the surface in some areas such as the Adirondacks, Thousand Islands, and regions of New York City (such as the Hudson Highlands and Central Park) due to erosion.

Left: Adirondack hills from the top of a watch tower in late fall. Right: a close-up of a gneiss rock surrounded by orange leaves.
Image 2: The Adirondacks as seen from Goodnow Mountain Watch Tower (left) and a Grenville Province gneiss found along the Goodnow Mountain Trail (right). The alternating layers of white and dark within the rock is the result of very high pressures and temperatures.  Photos courtesy of Alexandra Bancos.

Around 750 million years ago, Rodinia split apart and Laurentia became its own continent once again. By this point, the Grenville mountain range had almost completely eroded away.

575 - 455 million year ago: The Avalonian-Cadomian Orogeny

This period of time is geologically complicated and not completely understood. It involved more collisional and volcanic events that resulted from the break-apart of Rodinia and numerous other complicated interactions between the continents of the time. These events led to further metamorphism along New York’s coast and the addition of new igneous rock into the landscape.

More importantly, this period of time saw New York almost fully submerged under the ancient Potsdam Sea. Under this sea, much of the Cambrian Period (541-484 million years ago) and Ordovician Period (484-444 million years ago) sedimentary rock formed, some of which would be metamorphosed by later orogenic events. These metamorphosed sedimentary rocks are a large part of the Manhattan Prong region (underlying most of New York City).

455 - 66 million years ago: The Appalachians

Plus the Catskills, the split of Pangaea, and the Palisades

The formation of the Appalachians resulted from numerous orogenic events, all of which occurred after the Avalonian-Cadomian. The northern Appalachians formed from 5 distinct orogenies: the Taconic (460-450 million years ago), the Salinic (450-423), the Acadian (420-400), the Neoacadian (395-350 million years ago), and the Alleghanian (300-290 million years ago). These orogenies metamorphosed the sedimentary rock which formed during the Cambrian and Ordovician Periods; these are a large part of the Manhattan Prong underneath New York City.  The Acadian orogeny resulted in the formation of the Catskills: a mountain range composed of mostly metamorphosed sedimentary rock. The most recent orogenic event - the Alleghanian - was the result of the formation of Earth’s most recent (and most well-known) supercontinent: Pangaea. Pangaea broke apart approximately 200 million years ago. During Pangaea’s break up, the splitting tectonic plates caused magma to more easily reach the surface of the earth. Near New York City, this allowed for a large horizontal layer of magma to intrude between layers of older rock and cool to form a thick layer of rock (geological name: a “sill”) known as “the Palisades.” This sill is most visible along New Jersey’s side of the Hudson river as a near-vertical cliff exposed due to the erosion of surrounding rock.

The Palisades towering over a dock along the Hudson River. Although green-leafed trees obscure most of the lower section, the dark gray rock can be seen above them.
Image 4: The Palisades as seen from the Ross Dock Picnic Area in New Jersey. Take a moment to appreciate its size. Imagine how much magma must have poured into layers of older, sedimentary rock and cooled over time to form a structure this big. Photo courtesy of Alexandra Bancos. 
Overview: The Effect of Orogeny

Today, the North American continent stands alone. Its east coast is in a period of geologic calm and will not experience another orogenic event until the continents once again collide to form a future supercontinent. Its landscape, however, bears the scars (and mountains) of previous collisions.

Each orogenic event had a similar effect on New York’s geology. First: orogeny, and its resulting metamorphism, deformation, and igneous (volcanic) activity. Part of the effect of orogeny, which changed the weight and shape of the landscape, was the formation of faults: large cracks/fractures in the Earth’s crust. These cover New York’s surface. Second: the accretion (addition) of new, hard rock to the North American craton, expanding the North American continent Eastward. Last: the erosion of rock and the deposition of sediment which occurred in the geologic calm which followed an orogenic event.

2,600,000 - 10,000 years ago: The Last Ice Age

Finger Lakes, Niagara Falls, etc.

New York’s landscape has been most recently affected by the last Ice Age, a period of time during which glaciers periodically grew and advanced south and then melted and retreated north. The last glacial advance peaked ~20,000 years ago and, during this peak, ice covered almost all of New York. Periodic glacial advances and retreats softened New York’s topography, eroding higher regions and depositing sediments in lower regions. New York’s famous Finger Lakes are the result of glaciers gouging out and damming a series of streams, turning them into long lakes. The Thousand Island region was sculpted into its “knob (island) and hollow (water)” landscape by glaciers. The melting of the glaciers is responsible for releasing the large amounts of water which carved out the Niagara Escarpment and created Niagara Falls. Long Island is the result of glacial moraines (sediment which piles at the edge of advancing glaciers as they push southward) and outwash (sediment deposited by melting glacial water as they retreat) which accumulated within the ocean to the south of New York State.

Left image: Niagara Falls. The bottom half of the waterfall is obscured by a fog. Right image: Taughanook Falls. The waterfall is thin and taller than Niagara Falls but much less powerful.
Image 5: Niagara Falls (left), the most powerful waterfall in New York, was carved out by the large volume of water released from melting glaciers. Taughannock Falls (right), the tallest waterfall in New York, is only the tallest because the glaciers carved out steep valleys into which the Taughannock Creek abruptly falls. This creek feeds into Cayuga Lake: one of the Finger Lakes. Both waterfalls flow over and cut into sedimentary rocks which formed, in large part, from the eroded rock of previous orogenies during the Silurian and Devonian Periods. Photos courtesy of Alexandra Bancos. 

Part 2: The Geology of the New York City Region

Left image: illustration showing the bedrock geology of northern Manhattan and the Bronx. Right image: illustration of the bedrock geology of Staten Island.
Figure 1: Bedrock geology map of northern Manhattan and the Bronx (left) and Staten Island (right).
Surface geology map of New York City
Figure 2. Surface geology map of New York City showing the widespread deposition of glacial sediment which obscures the bedrock geology below.

There are a surprising number of areas in which the bedrock of New York City can be seen at its surface. Central Park is, perhaps, one of the best examples. This 843 acre green space is located over a region of metamorphic rock known as the Manhattan Prong which formed during the Grenville and Appalachian-forming orogenies. The Manhattan Prong underlies the entirety of the Bronx and Manhattan, the upper part of Staten Island, and the western edge of Brooklyn and Queens. Figure 1(left) illustrates the general types and locations of Manhattan Prong rocks, including those that formed during the Appalachian-forming orogenies - Hartland Schist (green), Inwood Marble (yellow), and Manhattan Schist (red) - and those that formed during the Grenville Orogeny: Fordham Gneiss (blue). Out of the 4 rock types, all except the Inwood Marble are visible on the surface in Central Park. You can see them yourself using this field guide made by the American Museum of Natural History. The metamorphic bedrock of Manhattan and the Bronx is overlain by a relatively thin layer of glacial sediment (see Figure 2).

The only bedrock in Brooklyn and Queens consists of a sliver of the Manhattan Prong. Above this is loose sediment from the Cretaceous Period (145-66 million years ago) and glacial deposits. The Cretaceous sediments consist of sand and clay and are known as the Raritan Formation (the same as that of Staten Island in Figure 1 (right)). The glacial deposit sediments are mostly gravel. Long Island, where the two boroughs are located, formed due to glacial sediment deposition from glacial moraines and outwash. Glacial moraines are large amounts of sediment which build up on the edges of glaciers as they push across a landscape. When the glacier begins to melt, these sediments are left behind. Glacial outwash occurs as glaciers melt and release large amounts of water. This water carries sediments across long distances and, as it slows, deposits them.

Staten Island’s bedrock consists of serpentinite which intruded into the Manhattan Schist of the region, shale and sandstone sedimentary rocks from the Triassic Period (252-201 million years ago), and the Palisades igneous intrusion. Above this bedrock is the Cretaceous Raritan formation, consisting of sand and clay sediments, and glacial sediment deposits. The Arthur Kill region consists of human-made landfills.

Most of New York City’s bedrock has been covered by sediments, artificial fill, and other human-made features, making it difficult to see the extent of bedrock. Despite this, geologists have been able to make accurate maps of the region’s geology using exposed rock outcrops and more complicated methods such as coring. Exposed outcrops are mostly found on the surface in places such as parks. Due to tunnels and deep wells, exposed rocks can also be found underground. Where underground rocks aren’t already exposed due to human-made features, geologists can look at “cores” - long, cylindrical columns of rock or sediment which have been extracted from the earth by a drilling mechanism in order to analyze subsurface, often very deeply located, rocks. Together, the two methods allow for a relatively complete picture of New York’s geology. Next time you find yourself out for a walk in this city, think about all the geologic layers under your feet and the hundreds of millions of years which formed them.